Should I do this without a dietitian?

Possible but harder. Phase 1 is the simple part; phase 2 requires structured reintroduction that's easy to do wrong without guidance. Studies show much higher success rates with a Monash-trained dietitian. NHS gastroenterology referral can include this; private dietitians charge £80-150 per session, usually 3-4 sessions over 3 months.

Will low-FODMAP harm my microbiome?

Phase 1 reduces Bifidobacterium because it cuts the fibres they ferment. This is why phase 1 is meant to be 4-6 weeks, not permanent. Phase 3, where you re-add tolerated foods, restores microbial diversity. Long-term very-restrictive low-FODMAP diets are not recommended.

Is low-FODMAP gluten-free?

Mostly, because wheat is high in fructans (a FODMAP). But it's the fructans, not the gluten, causing the IBS reaction in most non-coeliacs. Many people on low-FODMAP can tolerate sourdough wheat bread because the long fermentation breaks down fructans. Coeliac disease still requires strict gluten-free regardless.

What if low-FODMAP doesn't work?

About 30 percent of IBS patients don't respond. Worth investigating bile acid diarrhoea (especially if IBS-D), SIBO, microscopic colitis, or in-women cycle-driven symptoms. Stress and sleep also strongly influence IBS, sometimes the food isn't the main lever.

Food intolerance and FODMAPs

Low-FODMAP diet

Also called: FODMAP elimination diet, low FODMAP plan

The low-FODMAP diet is a 3-phase elimination plan developed at Monash University to identify which fermentable carbohydrates trigger IBS symptoms. Phase 1: cut FODMAPs strictly for 4-6 weeks. Phase 2: reintroduce one group at a time to find your specific triggers. Phase 3: settle into a long-term diet that limits only your personal triggers. About 70 percent of IBS patients improve. Best done with a registered dietitian.

Why it works for IBS

FODMAPs (fermentable oligo-, di-, mono-saccharides and polyols) are short-chain carbohydrates that the small intestine doesn't always absorb well. They reach the colon, where bacteria ferment them into gas and short-chain fatty acids. They also pull water into the small intestine through osmosis. In a normal gut this is fine. In IBS guts (with heightened visceral sensitivity), the gas and water cause more pain and bloating than they would in someone without IBS.

Phase 1, strict elimination (4-6 weeks)

Cut all high-FODMAP foods. The Monash FODMAP app is the gold-standard reference, updated as new foods are tested. Assess symptoms after the 4-6 week period. If they have not improved, FODMAPs probably aren't the trigger and it's worth considering other therapies.

Phase 2, structured reintroduction (6-8 weeks)

Reintroduce ONE FODMAP group at a time over about 3 days per challenge, keeping the background diet low-FODMAP, with rest days between groups. The 6 main groups are fructans (wheat, garlic, onion), galacto-oligosaccharides (legumes), lactose (milk products), fructose excess (honey, mango), sorbitol (apple, stone fruit), and mannitol (mushrooms, cauliflower). You'll likely tolerate some at full doses, others at moderate, and some not at all.

Phase 3, personalised long-term diet

Re-add everything you tolerated. Limit only your personal triggers, and only at the doses that cause symptoms. Most people end up with a less restricted diet than phase 1 but more controlled than they had before. Many also find that tolerance shifts over time, what triggered them in phase 2 may be fine 6 months later if their gut microbiome rebalances.

Common questions

Should I do this without a dietitian?: Possible but harder. Phase 1 is the simple part; phase 2 requires structured reintroduction that's easy to do wrong without guidance. Studies show much higher success rates with a Monash-trained dietitian. NHS gastroenterology referral can include this; private dietitians charge £80-150 per session, usually 3-4 sessions over 3 months.
Will low-FODMAP harm my microbiome?: Phase 1 reduces Bifidobacterium because it cuts the fibres they ferment. This is why phase 1 is meant to be 4-6 weeks, not permanent. Phase 3, where you re-add tolerated foods, restores microbial diversity. Long-term very-restrictive low-FODMAP diets are not recommended.
Is low-FODMAP gluten-free?: Mostly, because wheat is high in fructans (a FODMAP). But it's the fructans, not the gluten, causing the IBS reaction in most non-coeliacs. Many people on low-FODMAP can tolerate sourdough wheat bread because the long fermentation breaks down fructans. Coeliac disease still requires strict gluten-free regardless.
What if low-FODMAP doesn't work?: About 30 percent of IBS patients don't respond. Worth investigating bile acid diarrhoea (especially if IBS-D), SIBO, microscopic colitis, or in-women cycle-driven symptoms. Stress and sleep also strongly influence IBS, sometimes the food isn't the main lever.

Sources

Monash University - The 3 phases of the low FODMAP diet (restriction, reintroduction, personalisation) (Monash University)
NICE CG61 - Irritable bowel syndrome in adults: dietary advice to be given only by a healthcare professional with expertise in dietary management (NICE)
British Dietetic Association systematic review and evidence-based practice guidelines for the dietary management of IBS in adults (2016 update), McKenzie et al (J Hum Nutr Diet (PMID 27272325))
A diet low in FODMAPs reduces symptoms of irritable bowel syndrome, Halmos et al 2014 (Gastroenterology (PMID 24076059))
Effects of a low FODMAP diet on the colonic microbiome in IBS: systematic review with meta-analysis (Bifidobacteria reduction), So, Loughman, Staudacher 2022 (American Journal of Clinical Nutrition (PMID 35728042))
The Low-FODMAP Diet in Clinical Practice (3-phase structure, up to three-quarters respond, dietitian-led) - StatPearls (StatPearls, NCBI Bookshelf)
Evolution, adaptation, and new applications of the FODMAP diet (mechanism: osmosis, fermentation, visceral hypersensitivity), O'Brien et al 2024 (JGH Open (PMC11103764))